“It’s a significant role, we’re not throwing the role away,” Moffat told reporters during BBC America’s Television Critics Association Summer press tour panel. “We’re not just getting the star value and doing nothing with it.

“It’s a great part and she’s terrific in it,” he continued to the English actress who is best known for her role as Arya Stark on HBO’s “Game of Thrones.” “But, I would say, it will develop in unexpected ways.”

“One thing I think is really worth saying — because it keeps coming up as an issue — she is not playing a returning character, she’s a brand new character,” Moffat explained. “She’s not someone from the Doctor’s past.

“I can’t really tell you [much], but I think when you see what she’s up to in the show, you’ll appreciate what a clever idea [it is],” he teased reporters.

“Doctor Who” returns on Saturday, Sept. 19 on BBC America, marking titular star Peter Capaldi’s second season as the iconic Time Lord.

The first season of “Doctor Who” with Capaldi in the title role wound up being the highest-rated season of the series on BBC America. Overall, the season averaged 2.035 million viewers in Live + 3 Day ratings.

Along with Williams, the new season will also feature Tom Stourton, Ariyon Bakare, Simon Lipkin, Ian Conningham, Murray McArthur, Barnaby Kay, John Voce and Struan Rodger.

39 Spoiler-Tastic 'Doctor Who' 50th Anniversary Special Photos

The 11th Doctor (Matt Smith), the 10th Doctor (David Tennant), and the War Doctor (John Hurt)